Integration of different sarcoma virus genomes into host DNA: evidence against tandem arrangement and for shared integration sites.

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Abstract
Cellular DNA of 50-54 S was extracted from chicken embryo cells doubly infected with 2 different avian sarcoma viruses and was analyzed by the infectious DNA assay. Approximately 80-90% of the transformed foci that were induced by this DNA gave rise to 1 kind of avian sarcoma virus only, indicating that most proviral genomes are not integrated in tandem. When the 2 infecting viruses were varied with respect to multiplicity or time of infection, the initial infecting virus or the virus of higher multiplicity of infection was recovered at higher frequency in foci produced by the extracted DNA. This observation suggests the existence of common integration sites for different avian sarcoma viruses. Cells uniformly infected with avian leukosis virus could be transformed by superinfection with an avian sarcoma virus from a different envelope subgroup. Infectious DNA recovered from such cells contained 3-10 50% infectious dose (ID50) units of leukosis virus/.mu.g but only 0.3-0.4 ID50 of sarcoma virus. DNA from cells infected with sarcoma virus alone contained 3 sarcoma virus ID50/.mu.g. Even though a 2nd virus integrates with lower efficiency into preinfected cells, there probably is not a complete block of integration sites by the 1st virus.